IPAM: make max fixed IP validations DRY
IPAM: make max fixed IP validations DRY

Reuse code to make the validations DRY. In addition to this
fixed a typo.

Change-Id: I69cc904d30aae0e2d189a72c9dddaaab63a923ca

eayunstack/neutron,huntxu/neutron,apporc/neutron,dims/neutron,noironetworks/neutron,klmitch/neutron,chitr/neutron,openstack/neutron,bigswitch/neutron,mahak/neutron,MaximNevrov/neutron,apporc/neutron,chitr/neutron,noironetworks/neutron,huntxu/neutron,MaximNevrov/neutron,igor-toga/local-snat,cloudbase/neutron,mahak/neutron,igor-toga/local-snat,eayunstack/neutron,cloudbase/neutron,klmitch/neutron,mahak/neutron,wolverineav/neutron,openstack/neutron,bigswitch/neutron,dims/neutron,sebrandon1/neutron,sebrandon1/neutron,openstack/neutron,wolverineav/neutron